Length, comes from the Latin longus (long) can be defined as the maximum dimension of a body or flat figure.
In geography, it is the angular distance from a point on the earth's surface to the Greenwich meridian, determined by the arc of the equator between this meridian and the terrestrial point under study; It is measured in degrees, minutes and seconds up to 180.
There are several units of measurement that are used to measure length, and others that are obsolete. The units of measurement can be based on the length of different parts of the human body, on the distance traveled in number of steps, on the distance between reference points or known points on Earth, or arbitrarily on the length of a certain object.
According to the international system, the basic unit of length is the meter. The centimeter and kilometer are derived from the meter, and are commonly used units.
The units that are used to express distances in the immense are the light year, the parsec or the astronomical unit. On the contrary, to express very small distances, some units are; the micrometer, the ångström, the Bohr radius or the Planck length.
